An acoustic transducer is an electronic device used to emit and receive sound waves. Ultrasonic transducers are acoustic transducers that operate at frequencies above 20 KHz, and more typically, in the 1-20 MHz range. Ultrasonic transducers are used in medical imaging, non-destructive evaluation and other applications. The most common forms of ultrasonic transducers are piezoelectric transducers. In U.S. Pat. No. 6,271,620 entitled, “Acoustic Transducer and Method of Making the Same,” issued Aug. 7, 2001, Ladabaum describes microfabricated acoustic transducers capable of competitive performance compared to piezoelectric transducers. The basic transduction element of the microfabricated ultrasonic transducer (MUT) described by this prior art is a vibrating capacitor. A substrate contains a lower electrode, a thin diaphragm is suspended over the substrate and a metalization layer serves as an upper electrode. If a DC bias is applied across the lower and upper electrodes, an acoustic wave impinging on the diaphragm will set it in motion, and the variation of electrode separation caused by such motion results in an electrical signal. Conversely, if an AC signal is applied across the biased electrodes, an AC forcing function will set the diaphragm in motion, and this motion emits an acoustic wave in the medium of interest.
Capacitive transducers can transmit harmonics because the force on the diaphragm is proportional to the square of the applied voltage excitation waveform. Further non-linearity stems from the fact that the force on the diaphragm is also dependent, in a quadratic manner, on the position of the diaphragm relative to its resting state. Because broadband transducer designs have diaphragms that respond to such non-linear forcing functions in a meaningful manner, they transmit harmonics. Harmonic transmission from the transducer is undesirable in tissue harmonic imaging and contrast agent imaging because these imaging modalities are based on forming images with harmonics generated by the tissue or the contrast agent, not by the harmonics transmitted by a sub-optimal transmitter.
The use of pre-distorted input signals in electronic systems so as to reduce the harmonic content of an output signal is a technique that has been used in electronics for a long time and is well known in the art. For example, Holbrook et al., in U.S. Pat. No. 2,999,986 issued in 1961, teach a pre-distortion technique to reduce harmonics generated by a non-linear vacuum tube. Savord et al. received U.S. Pat. No. 6,292,435 for the application of pre-distorted signals to capacitive microfabricated ultrasonic transducers (cMUT). Fraser received U.S. Pat. No. 6,443,901 also for the application of pre-distorted signals to cMUTs. Hossack, in U.S. Pat. No. 6,461,299 teaches different pre-distortion methods to those taught in Savord et al. and Fraser. Savord et al., Fraser, and Hossack exclusively teach pre-distortion approaches to remove harmonics from the transmit signal. Pre-distortion approaches place design challenges on a system's transmitter. At best, they require a sophisticated and relatively expensive transmitter. At worst, the approach requires an entirely new ultrasound system to operate cMUTs in harmonic imaging mode.
A further significant disadvantage of the pre-distorted transmit signal approach is that it will not work in combination with the multi-polarity biased cMUT elements disclosed in the commonly owned U.S. patent application Ser. No. 10/367,106 filed Feb. 14, 2003, which has previously been incorporated by reference. For example, the appropriate pre-distortion for a positively biased cMUT is different from the appropriate pre-distortion for a negatively biased cMUT. Thus, when a transducer element contains both positive and negative bias regions, no single pre-distorted waveform can effectively cancel harmonic transmission from both the positive and negative bias regions. It is thus desirable to operate cMUTs for harmonic imaging in a manner that does not require pre-distorted transmit signals.
In U.S. Pat. No. 5,233,993, Kawano teaches a method whereby an ultrasound system forms an image based on the combination of two echoes from two transmit signals in the same scanning direction. In U.S. Pat. No. 5,632,277 Chapman et al. teach a method of generating an ultrasound image that enhances regions of non-linear scattering media by using two transmit signals 180 degrees apart in phase. In such an approach, the received echoes from linear media will be opposites of each other and cancel if added, but if a region is non-linear, there will be no significant difference in the received echoes of the harmonic energy. Further, Hwang et al., in U.S. Pat. Nos. 5,706,819 and 5,951,478, teach specifics of such an approach for imaging with contrast agents. Averkiou et al., in U.S. Pat. No. 6,186,950 introduces improvements to such pulse inversion harmonic imaging by using more than two transmit pulses per frame. U.S. Pat. No. 5,902,243 to Holley et al. and U.S. Pat. No. 5,961,463 to Rhyne et al. teach specifics of useful transmit waveforms. Common to all such prior art is that the only method taught for producing suitable transmit waveforms is to use the signal generator of the ultrasound system to distort, encode, or phase invert the transmit waveforms. It is therefore desirable to have a method for harmonic imaging that can relax or obviate these signal generator requirements of the ultrasound system's transmitter.
It has been realized by the present inventors that reversing the bias polarity of a cMUT is an effective means of introducing a 180 degree phase shift to the fundamental component of excitation waveform. When transmit signals are small compared to the bias voltage on a cMUT so that the cMUT is operating in a fairly linear range, the bias on the cMUT can be used to implement pulse inversion harmonic imaging with a simple transmitter.
It has been further realized by the present inventors that reversing the bias polarity of a cMUT introduces a 180 degree phase shift on the odd harmonics of a transmit waveform (i.e. if the fundamental is at frequency f, odd harmonics are at 3 f, 5 f etc.), but does not affect the even harmonics (i.e. 2 f, 4 f, etc.). Thus, such bias inversion can be used to enhance the harmonic image from media with scattering in the odd harmonic frequency range.
It has also been realized by the present inventors that a tight spatial distribution of alternating bias polarity across a cMUT element's aperture results in a transducer whose fundamental content is effectively canceled, but whose even harmonic content is the same as that of a cMUT with the same polarity bias across its aperture. Thus, the present inventors have discovered a mode of operating a cMUT in such a way that only its even harmonic content effectively radiates. When used in a method of multiple transmit firings and received signal combinations, this harmonic-only mode of operation can be used to remove the effects of cMUT generated harmonics. This is particularly valuable in tissue harmonic imaging or contrast agent imaging where high transmit powers are required, and where it is likely that cMUT generated harmonics will be significant.
It has been further realized by the present inventors that the focusing of the bias polarity, as disclosed in the commonly owned U.S. patent application Ser. No. 10/367,106 filed Feb. 14, 2003, is dispersive. This dispersion can be used to reduce transmitted second harmonic in the focal region of interest, then on receive, switch the spatial distribution of bias polarities to preferentially enhance the tissue generated harmonics received from the focal area of interest. Such dispersion is useful in general, not just for harmonic imaging, because it provides for a way of trading tightness of focus and depth of field via the frequency content of the transmit pulse.
Thus, the polarity of bias on the cMUT and the spatial distribution of such bias polarity patterns can modulate the phase and harmonic content of a cMUT's acoustic transmission. This bias-polarity-based modulation can be used to enhance the performance of cMUTs in harmonic imaging.
Further still, the combination of spatially distributed bias amplitude and polarity distributions with multiple transmit firings can be used to optimize the images rendered by cMUTs, whether harmonic or not.
Finally, in the course of experimentation, the present inventors have realized that periodically switching the bias polarity across a cMUT element can improve its performance with regard to degenerative processes such as charge trapping. It is thus desirable, even in the case were none of the advantages of harmonic imaging, elevation focusing, or elevation steering are relevant, that cMUTs be operated in such a manner to periodically reverse the polarity of the biasing electric field.